I had an idea about all this the other day. By third grade, most everyone can read a chapter book and kids have got definite preferences in their reading materials. This gives an appearance of evening out. If you don't look too closely (eg, comprehension of same book, time taken to read the same book, what books each kid is actually reading), it looks like everyone is around the same level.

It's like talking. Babies say their first words at different times and at, say, 18 months, a few will be yakking away, a few won't have said a word, and most will be somewhere in between to various obvious degrees. But by the time everyone is 6, nearly everyone speaks in paragraphs. So they sound a lot alike as long as you don't listen too closely.

Val